Hello there, welcome
Since the two you have in the cage are getting along harmoniously, I wouldn’t rock the boat (or cage
). As mentioned, an even number is best. Sometimes when budgies are youngsters it doesn’t seem to matter as much, but certainly upon sexual maturity, when territorial instincts kick in more, it usually does cause one to be the ‘third wheel’. Besides, your cage size is perfect for two.
Also, keep in mind the dynamics of individual personality, when thinking about bringing in new animals. No matter what size cage you have, there is never a guarantee budgies will get along with each other, and we must always have plan B, such as keeping other budgie(s) in another cage permanently, even after quarantine.
Another thing is, keeping a maximum number of budgies in any size cage, means a noticeable increase in poop, feathers, dust and debris to keep under control. The larger the cage with smaller number of budgies makes cleaning a bit more easily manageable. Budgies are messy little creatures, and even adding one makes a big difference.
The best thing for you at this point is to take advantage of the educational resources here at Talk Budgies. Look over the Stickies posts at the top of forum sections, and our Articles section. This will give you some useful and important information on keeping your budgies healthy and happy. The reading will give you some vital information, should you decide to get another cage and birds in the future.